Optical System Contamination: Effects, Measurements and Control VII

Assessment of thruster induced contamination of critical surfaces is the important and necessary stage of International Space Station (ISS) component design. Plume contamination models aimed at the prediction of the spatial distribution of contaminant mass fluxes have been developed in the recent years for various ISS thrusters based on vacuum chamber and space flight experiments and empirical correlations [1, 2]. Relatively scarce space flight contamination data have been gathered to date and are mostly limited to the measurements of thruster plume centerline contaminant fluxes. It is necessary, however, to have detailed knowledge of the contaminant deposition as a function of the angle from the plume centerline, especially, in the plume backflow region. The realization of this fact has motivated new flight experiments which are being planned for a future Space Shuttle mission [3]. Numerical modeling of thruster efflux transport and deposition on spacecraft surfaces can help to verify and improve the empirical plume contamination models. Strong rarefaction of plume backflow and atmosphere around ISS requires a molecular, kinetic approach to be used. The direct simulation Monte Carlo (DSMC) method based on the kinetic theory of gases has been successfully applied in the last decade for prediction of the molecular contaminant fluxes for various spacecraft plumes [4, 5] and outgassing sources [6]. A continuum approach for the internal part of the nozzle and the DSMC method for plume nearfield was used in [7], where propellant droplets where modeled using a Lagrangian particle tracking. The importance of droplets originated at the cooling film in the region of the nozzle lip was also emphasized in [7]. The principal goal of the present work is to examine both particulate and molecular contaminant transport for a 130 N bipropellant thruster, a typical small thruster used on ISS, and analyze their relative importance near the plume axis as well as in the back flow region in order to provide useful data for comparison with ground-based and space flight